Field of the Invention
The invention relates to a receiving device for receiving a magnetic field and for producing electric energy by magnetic induction, in particular for use but not limited to the use by a vehicle. The invention also relates to a method of manufacturing such a receiving device and to a method of operating such a receiving device. The invention can be applied in the field of wireless transfer of energy, in particular to vehicles, such as road automobiles, busses and rail vehicles. In particular, the receiving device can be adapted to produce electric power at least at rates in the kilowatt range, e.g. 20 kW.
Description of Related Art
GB 2501482 A describes an arrangement for providing a land vehicle, in particular a rail vehicle or a road automobile, with electric energy by producing an electromagnetic field on a primary side located on the track of the vehicle and/or located at a stop of the vehicle, by receiving the electromagnetic field on a secondary side onboard the vehicle above the source of the electromagnetic field and by magnetic induction on the secondary side. A secondary side conductor assembly is made of electrically conducting material which produces an electromagnetic stray field during operation while the electrically conducting material carries an alternating electric current. A secondary side shielding assembly made of magnetizable material shields regions, which are located beyond the magnetizable material, from the electromagnetic stray field.
As described in GB 2501482 A, the secondary side conductor assembly and the secondary side shielding assembly may be parts of the same receiving device for receiving the magnetic field from the primary side. For example, the present invention may relate to a receiving device as disclosed by GB 2501482 A, which also comprises a capacitor arrangement comprising at least one capacitor, which is electrically connected to the conductor assembly.
In an inductive power transfer (IPT) system, the transfer of electric power is performed using two sets of windings of electric conductors, e.g. three-phase windings. A first set is installed on the primary side (e.g. on the ground) and can be fed by a wayside power converter. The second set of windings (comprising at least one coil of an electric line) is installed on the secondary side, such as on the vehicle. For example, the second set of windings can be attached underneath the vehicle. The device comprising the second set of windings is often referred to as pickup or receiver. In particular, the first set of windings and the second set of windings form a medium frequency (e.g. in the range of 1 kHz to 200 kHz) transformer to transform electric energy to the secondary side. This can be done in a static state (when there is no motion between the primary and the secondary side) and in a dynamic state (when the secondary side, such as the vehicle, moves).
Due to a clearance or gap, which is comparatively large compared to conventional transformers, between the primary windings and the secondary windings, the operational behaviour of this transformer differs from the behaviour of conventional transformers which have a closed magnetic core with negligible or small air gaps. The large air gap results in smaller mutual inductive coupling and larger leakage inductances.
To be able to transfer energy at high power levels, a capacitor arrangement is used in order to compensate the reactance of the inductors at the operating frequency e.g. in the middle frequency region. The combination of the inductance and the capacitance forms a resonant circuit. A perfect impedance cancellation happens if impedance values of the inductance and the capacitance are chosen such that the resonance frequency of the resonant circuit is equal to the frequency of an alternating electric current through the primary side windings. Such a resonant circuit is tuned.
In short, the purpose of the capacitor arrangement of the secondary side receiver is to form an electric circuit having a resonance frequency which is defined by the capacitance of the capacitor arrangement and by the inductance of the windings.
During operation of the system, significant changes of the receiver's electric properties have been observed. This refers to the electric properties of the electric lines and of the capacitor arrangement, including the inductance of the windings and the capacitance of the capacitor arrangement, as well as the magnetic properties of the magnetizable material. These changes of the electric properties affect the resonance frequency.